Field of the Invention
The present invention relates to a failure determination system for determining a failure of a vehicle speed detection device.
Description of the Related Art
Conventionally, there has been known a failure determination system for a vehicle speed detection device, disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2009-128239. This failure determination system is applied to a four-wheel vehicle of a rear-wheel-drive type, and includes a vehicle speed sensor for detecting a rotational speed of an output shaft of a transmission, four wheel speed sensors for detecting rotational speeds of the four wheels, respectively, an ABS (Anti-lock Brake System)-ECU connected to the sensors, and so forth. In the ABS-ECU, ABS control is performed based on detection signals from the four wheel speed sensors, in order to avoid occurrence of a locked state of the wheels.
Further, in the failure determination system, a first vehicle speed is calculated based on a detection signal from the vehicle speed sensor, and a second vehicle speed is calculated based on detection signals from two rear wheel speed sensors. In a case where predetermined failure determination conditions are satisfied, when the absolute value of a vehicle speed difference, which is a difference between the first vehicle speed and the second vehicle speed, is larger than a predetermined value, it is determined that the vehicle speed sensor is faulty.
In the recent functional safety standards (ISO26262), it is required to always monitor whether or not the vehicle speed sensor is faulty. To satisfy this condition, when the failure determination system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2009-128239 is applied, two wheel speed sensors are required in addition to the vehicle speed sensor, since a failure of the vehicle speed sensor is determined by the above-described method. However, vehicles used e.g. in developing countries are each provided with a single vehicle speed sensor for detecting a vehicle speed thereof, but a large number of them have no wheel speed sensors other than the vehicle speed sensor, because of lower prices. When the above-described failure determination system is applied to such vehicles, it is required to add two wheel speed sensors, which causes increases in manufacturing costs and product prices, resulting in degraded marketability.